The term thin walled component is well known and is understood to mean a structure in which a thickness of a wall of the structure is small in comparison to other dimensions (length, width, diameter, etc.) of the component.
A gas turbine engine casing is conventionally manufactured from a single piece forging. This manufacturing technique often results in the finished component having a Buy-to-Fly ratio of around 10. In other words, the weight of the raw forging can be approximately 10 times that of the finished machined casing.
The requirement to add projections to either the inner surface or the outer surface of the engine casing can significantly increase the Buy-to-Fly ratio because considerably more material must be added to the raw forging than will be required for the finished projection itself.
Adding such features using conventional welding techniques may result in high levels of distortion in the thin walled component. While it might be possible to prevent this distortion by increasing the section of the thin walled component this would result in a finished component of greater mass and therefore lower weight efficiency.
Alternatively, the whole of the thin walled component might be pre-heated to minimise distortion. This would require a significant amount of heat energy together with an enclosure sufficiently large to accommodate the thin walled component.
Statements of Disclosure
According to a first aspect of the present disclosure there is provided a method of attaching a projection to a thin walled component, the thin walled component having a first surface and an opposite second surface, the method comprising the steps of:                (i) providing a thin walled component;        (ii) identifying a first position on the first surface of the thin walled component;        (iii) building a hollow thin walled lattice structure on the second surface at a second position using a liquid metal deposition process, the second position corresponding to the first position; and        (iv) building a projection on the first surface at the first position using a liquid metal deposition process.        
The building of a lattice thin walled lattice structure on one side of the thin walled component will significantly increase the local structural rigidity of the thin walled component.
The use of a liquid metal deposition process to build a projection on a second opposite side of the thin walled component minimises the quantity of heat energy added to the material of the thin walled component. This in combination with the increased local structural rigidity of the thin walled component considerably reduces any thermal distortion of the thin walled component by the addition of the projection.
Optionally, the method comprises the further step of:                (v) removing the hollow thin walled lattice structure from the second surface.        
In one arrangement of the disclosure, the thin walled lattice structure is removed after the projection has been built. This results in the thin walled component together with the attached projection, without the need to machine away excess material to achieve this resulting finished component. This makes the arrangement both cost and time efficient compared to prior art alternatives.
The lattice structure may be removed by any suitable material removal technique such as, for example, electro-deposition machining. Any such technique must not remove material corresponding to the wall of the thin walled component.
In another arrangement of the disclosure, the thin walled lattice structure is retained, where the volume occupied by the lattice structure is not required for another purpose. This can results in a local increase of the structural rigidity of the thin walled component, which, in turn, may allow the size of the projection to be reduced so further increasing the weight efficiency of the thin walled component.
Optionally, the method comprises the further step of:                (vi) heat treating the thin walled component to relieve residual stresses resulting from the addition of the projection.        
The use of heat treatment to relieve residual stresses resulting from the addition of the projection may further improve the dimensional stability of the thin walled component.
Optionally, the hollow thin walled lattice structure comprises a plurality of elongate cells, each of the cells having an axis that extends substantially normally to the second surface.
The thin walled lattice structure is built up from the second surface of the thin walled component to form an arrangement of adjoining cells.
In one arrangement, the cells have a hexagonal cross-sectional shape to provide for an efficient space-filling arrangement. A hexagonal cross-section for the cells provides the most efficient shape in terms of strength and weight.
In other arrangements, the cells may have other cross-sectional shapes such as, for example, square, diamond or triangular, that also provide for a space-filling arrangement.
Optionally, each of the cells has a wall thickness of between 0.5 mm and 1.0 mm.
This minimises the quantity of material required to build the lattice structure so making the method more time and cost efficient.
Optionally, method step (iv), comprises the concurrent step of:                (iv-a) during the building of the projection, directing a cooling fluid over the hollow thin walled lattice structure to impinge upon the second surface of the thin walled component.        
The cooling of the thin walled component by the directed flow of a cooling fluid impinging against the second surface of the hollow lattice structure may reduce any distortion of the thin walled component resulting from the heat input from the liquid metal deposition process.
Optionally, method step (iv), comprises the concurrent step of:                (iv-a′) during the building of the projection, directing a heating fluid over the hollow thin walled lattice structure to impinge upon the second surface of the thin walled component.        
In an alternative arrangement, a heating fluid is directed onto the second surface of the hollow lattice structure to thereby offset the effect on the dimensional stability of the thin walled component caused by the heat input from the liquid metal deposition process.
Optionally, method step (iii) comprises the further step of:                (iii-a) filling the hollow thin walled lattice structure with a filler material.        
As an alternative to the directed flow of a heating fluid impinging against the second surface of the thin walled component, the hollow thin walled lattice structure may be filled with a filler material.
The use of a filler material to fill the cells of the lattice structure will increase the structural rigidity of the lattice structure by increasing the buckling limit of the lattice cell walls.
The addition of the filler material will reduce the thermal energy transfer from the second surface through the lattice structure. This in turn may reduce the thermal gradient in the projection as it is built onto the first surface of the thin walled component.
Optionally, the filler material is selected from the group consisting of ceramic slurry and ceramic foam.
Ceramic slurry and ceramic foam materials may be readily added to the cell voids of the lattice structure, and provide the increases in structural rigidity and insulation that are discussed above.
Optionally, the projection is selected from the group consisting of boss, tube, flange, bracket and stub.
The projection may be any form of three-dimensional geometrical feature, such as a boss, a tube, a flange, a bracket or a stub.
According to a second aspect of the present disclosure there is provided an article comprising a thin walled component having a projection attached thereto by the method according to the first aspect.
The method of the disclosure is applicable to the addition of any three-dimensional feature to a generic thin walled component. Examples of such component include pressure vessels, tanks, etc.
According to a third aspect of the present disclosure there is provided a gas turbine engine casing comprising a thin walled component having a projection attached thereto by the method according to the first aspect.
A gas turbine engine comprises an engine casing in the form of thin walled component as hereinbefore defined and discussed.
Other aspects of the disclosure provide devices, methods and systems which include and/or implement some or all of the actions described herein. The illustrative aspects of the disclosure are designed to solve one or more of the problems herein described and/or one or more other problems not discussed.
It is noted that the drawings may not be to scale. The drawings are intended to depict only typical aspects of the disclosure, and therefore should not be considered as limiting the scope of the disclosure. In the drawings, like numbering represents like elements between the drawings.